Support is requested to set-up a sophisticated colony of Syrian hamsters for the purpose of raising new isogenic lines from recent catches of outbred wild hamsters, of producing congenic resistant lines both within the several inbred strains domestically maintained and by introducing genes from the wild population into the well-known laboratory strains. The objective of producing these new lines of animals will be to determine the characteristics of the major histocompatibility complex in this species. Recent evidence suggests that the MHC of hamsters may be aberrant in that the expected array of cell surface antigens dictated by genes generally found at this region are not present, that genes governing immune responsiveness to unrelated antigens and governing levels of serum complement activity are unlinked to the genes governing skin graft rejection, mixed lymphocyte reactivity and graft-versus host reactivity. Moreover, evidence of markedly restricted polymorphism for the hamster major histocompatibility complex is also unusual. The new strains to be created will be analyzed for evidence of degree of polymorphism at the MHC, extent of recombination within the genes that are linked in the hamster MHC, the rate of mutation of genes found within this region, and the relationship of this genetic region to immune responsiveness, viral susceptibility and oncogenesis.